In 1815, following the Napoleonic Wars, the powers of Europe had created out of the lands previously called the Austrian Netherlands a new country called Belgium, whose neutrality was guaranteed by, among others, Britain.
In August of 1914, Germany claimed that France was preparing to invade Belgium to attack Germany. "Requesting" free passage across Belgium to meet the French, Germany threatened war if the Belgian army resisted. On 3rd August, 1914, Germany invaded. By the end of the year, virtually all of Belgium was held by Germany. Only the small strip of coast between Ypres and the French border was held. The Belgian king moved his court to Nieupoort, on the French border and remained there for the duration of the war.
Operating behind German lines. Belgian snipers, or Franc-Tireurs (literally, "free-shooters"), caused some damage to German troops before brutal reprisals against the civilian population forced the Belgian army to pull them out.
Meanwhile, in Africa, Belgian colonial troops from the Belgian Congo fought the Germans in East Africa alongside the British and South African forces.
Total Belgian military casualties of the war are estimated at 44,000 although the number of civilian casualties is not included in that figure.

Belgian Garde Civique in Liege, 1914.

The Belgian King being welcomed home by his subjects

Belgian Congolese troops in Africa